The problem is the Tesla app is not requesting the Calendar access when it doesn't have it. And since the app was freshly installed on a new phone, the phone doesn't know the app needs Calendar access (iOS bug). Reinstalling the app makes it ask again.
This can be fixed by Tesla by having their app request calendar access if it needs it, even if it isn't a fresh install. A button in the options screen would be a good way to do this.